1. Field of the Invention
The present invention relates to a switching power supply apparatus which performs switching control using digital control circuits.
2. Description of the Related Art
One type of protection circuit which is generally provided in a switching power supply apparatus is an overcurrent protection circuit. An overcurrent protection circuit detects a current flowing through a main switching element and turns off the main switching element when the current value reaches a predetermined value (see, Japanese Unexamined Patent Application Publication No. 2005-312139).
FIG. 1 illustrates an example of a switching power supply apparatus in Japanese Unexamined Patent Application Publication No. 2005-312139.
In the switching power supply apparatus illustrated in FIG. 1, a series circuit composed of a primary winding 2a of a transformer 2 and a semiconductor switching element 3 is connected to opposite ends of a direct current power supply 1. A diode 4 is connected to a secondary winding 2b of the transformer 2. Energy accumulated in the transformer 2 during an ON period of the switching element 3 is supplied to a load via the diode 4 during an OFF period of the switching elements 3. A current flowing through the switching element 3 is converted into a voltage signal by a resistor 6. The voltage signal is rectified by a diode 10, and the voltage is held for a predetermined time period by a charge-discharge circuit composed of a capacitor 11 and a resistor 12. When a comparator 7 and a reference voltage generator 8 detect that the held voltage is equal to or higher than a predetermined value, a control circuit 9 turns off the switching element 3.
By arranging the overcurrent protection circuit in this way, magnetic saturation of the transformer can be prevented. In addition, overcurrent during starting and output short circuit can be prevented without solutions such as an increase in size of the magnetic core of the transformer 2 for preventing saturation and an increase in capacity of the switching element 3.
Meanwhile, in general, when switching control is performed using an analog control circuit, a predetermined voltage signal is compared with a reference voltage by a comparator. Then, depending on the large-small relationship with the reference voltage, switching control is carried out. However, in the case of such analog control, output of the comparator may instantaneously vary due to the influence of switching noise superimposed on a voltage/current signal, which may result in unstable control. Insertion of a low-pass filter or the like for suppressing the influence of noise may lead to the problem of response delay.
The problem of noise influence and the problem of response delay caused by insertion of a low-pass filter arise also in a case where an overcurrent protection circuit is composed of an analog control circuit as in the case of the switching power supply apparatus in Japanese Unexamined Patent Application Publication No. 2005-312139 illustrated in FIG. 1.
The above problems may be overcome if switching control is performed by a digital control circuit. However, in order to detect overcurrent using a digital control circuit, a current flowing through a path to an object to be protected from overcurrent is converted into a voltage signal. Then, the voltage signal is sampled, and the absolute value of the sampled voltage signal is calculated. This process takes a long time to calculate the absolute value to be compared with a threshold value and leads to another problem of delay of determination of the presence or absence of magnetic saturation of a transformer or an inductor.